1. Field of the Invention
This invention relates to implantable medical devices and methods of making such devices that have selected regions with different material properties than other regions of the device.
2. Description of the State of the Art
This invention relates to generally to medical devices having regions with different clinical or therapeutic and mechanical requirements. Therefore, the invention may be applied to a diverse array of medical devices, including, but not limited to radial expandable endoprostheses, heart valves, bone screws, and suture anchors. For example, radially expandable endoprostheses are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involved compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
The structure of stents is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed to allow the stent to be radially expandable. The pattern should be designed to maintain the longitudinal flexibility and radial rigidity required of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen.
Stents have been made of many materials such as metals and polymers, including biodegradable polymer materials. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers may be configured to meet this additional clinical requirement since they may be designed to completely erode after the clinical need for them has ended.
Conventional methods of constructing a stent from a polymer material involves extrusion of a polymer tube based on a single polymer or polymer blend and then laser cutting a pattern into the tube. A potential shortcoming of such methods is that material properties of the stent formed by such methods tend not to vary substantially throughout different regions of the stent. Conventional methods of fabricating metallic stents suffer from the same shortcoming. Material properties may include mechanical and thermal properties. Additional material properties may also include the absorption rate of a biodegradable stent material and the composition of polymer and active agents or drugs impregnated into a polymer scaffolding. In addition, conventional coating methods also tend to form coatings that have relatively uniform material properties along the surface of a stent.
Due to certain mechanical and clinical requirements, it may be advantageous for certain material properties to be different in different portions of an implantable medical device. It may be desirable for some portions of a stent to have some mechanical properties, absorption rates, and composition different than in other portions of the device. For example, as indicated above, localized regions of a stent may have different mechanical requirements due to relatively high stress and strain during use, and, thus may require different mechanical properties for the different regions.